What is microgravity and what is its affect on humans?

Well basically, anything that has mass exerts a gravitational pull ony other masses. Therefore, when in space in a space station/ship, you are not actually experiencing weightlessness, but the gravitational effects of the ship are small enough as to not be noticable. This is called microgravity. As to its effects on humans, basically we experience muscle atrophy, or the wasting away of our muscles. The reason for this is that humans have evlolved in a gravitational environment where our muscles have developed due to the resistance of gravity. Therefor when spending long periods in space, we lose this conditioning. A classic image from the MIR space station was of the Russian cosmonaut being lifted out of his reentry capsule because his muscles had wasted to the point that he couldn't stand in (resist) Earths gravity.